Anisotropic failure behaviour and breakdown pressure interpretation of hydraulic fracturing experiments on shale

Shales with a laminated structure typically exhibit inherent anisotropy and strong brittleness, resulting in complex failure patterns and fracture evolution during hydraulic fracturing. To investigate the fracturing mechanism and layer effect of Longmaxi shale from southwest China, a novel testing setup with a high-efficiency sealing method was developed for laboratory hydraulic fracturing, based on which a series of fracturing tests was conducted on shale samples under different confining pressures. The anisotropic mechanical behaviour due to the shale lamination effect was considered by drilling samples at seven different inclinations with reference to the bedding plane. The results showed apparent brittle failure characteristics and anisotropic phenomena both in strength and failure patterns. Three main failure patterns can be identified as split, split-bending and slip along the bedding plane. The corresponding fracture roughness was further analyzed by laser scanning, which shows that the slip mode with shale layer activation can provide a relatively smooth channel. Of particular interest is that anomalously high breakdown pressure, which fluctuates in the samples at different layer orientations and cannot be explained by conventional models, was also observed in our fracturing tests. Inspired by the nonlocal treatment of line stress criterion, an average tensile stress-based model by introducing an equivalent characteristic length was then proposed for interpreting such unusual breakdown pressures, and the inherent mechanism behind hydraulic fracturing was discussed. Comparison analysis showed that the proposed criterion not only predicts the testing data quite well, but can also be degenerated to conventional models in specific cases. The knowledge achieved in this study may provide practical implications for perforation design and breakdown pressure evaluation in field hydraulic fracturing operations.